This invention relates to phosphated supports for chromium containing olefin polymerization catalysts. This invention also relates to a process to polymerize olefins.
Supported chromium oxide catalyst systems can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Supported chromium oxide catalyst systems can also be used to prepare olefin polymers in a slurry system wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has the advantage of being less complex. However, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form process. For instance, in the solution process, control of the polymer molecular weight can be effected by changing the temperature, with lower molecular weight (higher melt flow) being obtained at the higher temperature. However, in the slurry process, this technique is inherently limited since any efforts to increase the melt flow to any appreciable extent by increasing temperature would cause the polymer to go into solution and thus destroy the slurry or particle form process. Also, it is frequently desired to have a polymer with a broader molecular weight distribution than is normally obtained in the slurry or particle-form process in order to facilitate blow molding of the olefin polymer.
It is known in the art that the activity of such chromium oxide catalyst systems can be improved by treating the support with phosphorus-containing compounds. However, the resultant catalyst system usually tends to contain less phosphorus than a theoretical calculation would indicate should have been added. Additionally, it is desired to increase the surface area of such a catalyst system so as to improve potential productivity. Known phosphorus-containing catalyst system are irregularly shaped, i.e. not spherical, making them more difficult to transport and yielding an inferior polymer. It is also desired to have a catalyst system which is easy to manufacture.